Rhodium-catalyzed asymmetric hydroarylation of diphenylphosphinylallenes with arylboronic acids

Article abstract of DOI:10.1021/ja058763f

Rhodium-catalyzed asymmetric hydroarylation of diphenylphosphinylallenes with arylboronic acids proceeded in high yields with high regio- and enantioselectivity to give chiral allylphosphine oxides of up to 98% ee. The structural determination of the key

Full text of DOI:10.1021/ja058763f

Published on Web 02/03/2006
Rhodium-Catalyzed Asymmetric Hydroarylation of Diphenylphosphinylallenes
with Arylboronic Acids
Takahiro Nishimura, Sho Hirabayashi, Yuichi Yasuhara, and Tamio Hayashi*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan
Received December 26, 2005; E-mail: thayashi@kuchem.kyoto-u.ac.jp
Table 1. Rhodium-Catalyzed Asymmetric Hydroarylation of 1:
Solvent Effect
Recently, it has been disclosed that chiral rhodium complexes
efficiently catalyze the asymmetric addition of arylboronic acids
to activated alkenes offering one of the best ways to construct a
new C-C bond enantioselectively.1,2 In this context, we next
focused on the asymmetric arylation of allenes. It has been known
that arylmetal species add to allenes to generate π-allylmetal
species.3 As shown in Scheme 1, it would be possible to create a
stereogenic carbon center if the chiral π-allylrhodium intermediate
(I), formed by the reaction of a 1,1-disubstituted allene and an
arylrhodium species, undergoes the protonation regioselectively as
well as enantioselectively.4 This asymmetric transformation leading
to chiral alkenes (II) is difficult due to the competitive formation
of internal achiral ones (III), and to the best of our knowledge,
there have been no reports on the catalytic asymmetric hydroary-
lation of allenes.5 Here we report that it is realized by rhodium-
catalyzed addition of arylboronic acids to allenes bearing phosphine
oxide,6 which produces chiral allylic phosphine oxides of high
enantiomeric purity.
temp
C)
yield (2a
+
(%)a
3a)
ee (%)
of 2ac
entry
solvent
(
°
2a/3ab
1
dioxane
dioxane
THF
100
60
92
92
97
96
75/25
71/29
96/4
89
90
97
97
2
3
60
4d
THF
60
96/4
a Isolated yield. b Determined by 1H NMR. c Determined by HPLC
analysis with chiral stationary phase column: Chiralcel OJ-H. d [Rh(O-
H)(cod)]2 (5 mol % of Rh) and (R)-binap (5.5 mol %) were used instead of
[Rh(OH)((R)-binap)]2.
Table 2. Rhodium-Catalyzed Asymmetric Hydroarylation of
Diphenylphosphinylallenes: Scope of Arylboronic Acids and
Allenes
Scheme 1
time
(h)
yield
(%)a
ee
entry
R
Ar
(%)b
Phosphinylallenes are stable compounds and readily available
from chlorophosphines and propargylic alcohols in one step.7 Our
initial studies were focused on the asymmetric hydroarylation of
3-(diphenylphosphinyl)-3-methyl-1,2-butadiene (1) in the presence
of chiral rhodium complexes directed toward the catalytic synthesis
of chiral allylic phosphine oxide 2a, which is a useful intermediate
for chiral tertiary allylphosphines or chiral alkenes (Table 1).8,9
Treatment of allene 1 with p-tolylboronic acid (2 equiv) in the
presence of [Rh(OH)((R)-binap)]210 (5 mol % of Rh) in 1,4-dioxane
at 100 °C for 1 h gave the hydroarylation product 2a with 89% ee
together with a considerable amount of 3a (total yield 92%, 2a/3a
) 75/25, entry 1). The reaction temperature did not have a
significant influence on the selectivity (entry 2), but the change of
the solvent to THF markedly improved the regioselectivity as well
as the enantioselectivity, giving 2a with 97% ee (total yield 97%,
2a/3a ) 96/4, entry 3). [Rh(OH)(cod)]2 as a catalyst precursor can
be used as well in combination with (R)-binap (entry 4).
1
2c
3
1: Me
1: Me
1: Me
1: Me
1: Me
1: Me
1: Me
4: Et
p-MeC6H4
p-MeC6H4
p-MeOC6H4
C6H5
2-naphthyl
p-ClC6H4
p-CF3C6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
1
6
2a: 85
97 (S)
97 (S)
96 (S)
96 (S)
98 (S)
97 (S)
96 (S)
97 (S)
96 (S)
69 (S)
2a: 88
2b: 89
2c: 90
2d: 94
2e: 91
2f: 85
1
4
1
5
1
6
5
7
12
1
8
5: 94
7: 88
9: 40, 10: 45
12: 91
9
6: Bu
8: Ph
11: t-Bu
1
10
11
1
1
a Isolated yields after removal of a small amount of internal alkenes by
column chromatography on silica gel. b Determined by HPLC. Absolute
configuration of 2e was determined by X-ray crystallographic analysis. For
others, they were assigned by consideration of the stereochemical pathway.
c 1 mol % of Rh was used.
As shown in Table 2, aryl groups substituted with either electron-
donating or -withdrawing groups were introduced onto allene 1 with
high enantioselectivity to give the corresponding allylic phosphine
oxides 2a-2f with 96-98% ee (entries 1-7). A high yield of 2a
was obtained without loss of the enantioselectivity even in the
presence of a reduced amount of the catalyst (1 mol %) (entry 2).
The configuration of 2e was determined to be S by X-ray
crystallographic analysis.11 High enantio- and regioselectivities were
also observed for allenes 4 and 6, which are substituted with ethyl
and butyl, respectively (entries 8 and 9). On the other hand, allene
8 bearing a phenyl group gave 9 in a low yield with lower
enantiomeric excess (40% yield, 69% ee), internal alkene 10 being
accompanied in 45% yield (entry 10). A bulkier substituent (tert-
butyl) on 11 completely changed the regioselectivity to give 12 in
91% yield (entry 11).
9
2556
J. AM. CHEM. SOC. 2006, 128, 2556-2557
10.1021/ja058763f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
ized. We succeeded in the structural determination of the π-allyl-
rhodium intermediate to establish the catalytic cycle of the reaction.
Acknowledgment. This work has been supported in part by a
Grant-in-Aid for Scientific Research, the Ministry of Education,
Culture, Sports, Science and Technology, Japan (Priority Areas
“Advanced Molecular Transformations of Carbon Resources”).
Supporting Information Available: Experimental procedures and
spectroscopic and analytical data for the substrates and products (PDF)
and X-ray data files (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
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Figure 1. ORTEP illustration of η3-allylrhodium(I) complex 14 with
thermal ellipsoids drawn at 50% probability (hydrogens are omitted).
Scheme 2
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NMR studies on a sequence of stoichiometric reactions starting
from phenylrhodium complex [RhPh(PPh3)((R)-binap)]10 (13)
(Scheme 2) provided us with a significant insight into the catalytic
cycle of the present reaction. Treatment of 13 with 1 (1.0 equiv) in
THF-d8 at 60 °C for 30 min brought about selective formation of
a new rhodium complex. 31P NMR of the reaction mixture consisted
of two dd’s, 34.9 (JP-P ) 13, JRh-P ) 3 Hz), 46.9 (JRh-P ) 198
Hz, JP-P ) 35 Hz), and one ddd 38.4 (JRh-P ) 197 Hz, JP-P ) 35,
13 Hz) as well as a peak of free PPh3, which is consistent with
formation of π-allylrhodium complex 14. The structure of 14 was
successfully determined by X-ray crystallographic analysis.12 As
shown in Figure 1, two phosphorus atoms (P(2), P(3)) of (R)-binap
and π-allyl carbons (C(1), C(3)) constitute square planar orientation
around the Rh center. The diphenylphosphinyl substituent on the
π-allyl is located anti with respect to the phenyl group on the central
carbon C(2). This orientation is probably due to the steric effect
by one of the phenyl rings of the binap ligand. The absolute
configuration of the π-allyl moiety in 14 is 2R,3R. Addition of
phenylboronic acid (2.0 equiv) to the THF-d8 solution containing
14 and PPh3 gave, after heating at 60 °C for 30 min, hydrophe-
nylation product 2c and phenylrhodium complex 13. The isolated
2c (93% yield) was an S isomer of 96% ee, which is the same as
that obtained in the catalytic reaction.
Thus, we succeeded in establishing the catalytic cycle which
involves the addition of an arylrhodium species to allene, forming
a π-allylrhodium species, and protonolysis13 of the π-allylrhodium,
giving hydroarylation product, followed by transmetalation, regen-
erating the arylrhodium intermediate. The same stereochemical
outcome (96% ee S) observed in the catalytic reaction and the
stoichiometric reaction imply that the protonation in the catalytic
system occurs after the equilibration into a thermodynamically stable
π-allylrhodium intermediate (such as 14), and the R configuration
at C(3) of the π-allyl complex 14 indicates that the π-allyl undergoes
protonation from the same side as rhodium, although the detailed
mechanism of the protonation remains to be clarified.14
(10) Hayashi, T.; Takahashi, M.; Takaya, Y.; Ogasawara, M. J. Am. Chem.
Soc. 2002, 124, 5052.
(11) The Flack parameter is -0.03(5) with this configuration: Flack, H. D.
Acta Crystallogr., Sect. A 1983, 39, 876.
(12) For examples of the synthesis of π-allylrhodium(I) complexes from
rhodium complexes and allenes, see: (a) Osakada, K.; Choi, J.-C.;
Yamamoto, T. J. Am. Chem. Soc. 1997, 119, 12390. (b) Choi, J.-C.;
Osakada, K.; Yamamoto, T. Organometallics 1998, 17, 3044.
(13) The arylation of 1 with p-tolylboroxine in the presence of D2O gave (2-
deuterio-3-p-tolylbut-3-en-2-yl)diphenylphosphine oxide 2a(D) (82% D)
in 91% yield.
(14) A diphenylphosphinyl group may play an important role in the hydroary-
lation of allenes to obtain the desired adduct selectively. For example,
the reaction of 1-methyl-1-(trimethylsilyl)allene with p-tolylboronic acid
did not give the desired product at all under the same reaction conditions.
In summary, highly enantioselective hydroarylation (up to 98%
ee) of diphenylphosphinylallenes with arylboronic acids was real-
JA058763F
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